Cardiac output is the volume of blood pumped by the heart over a given time period. Cardiac output may be divided by body surface area to account for the size of a patient. Cardiac output may be used to assess the state of a patient's circulation. Simple measurements, such as heart rate and/or blood pressure, may be adequate to understand cardiac output for some patients, but more detailed measurements may be required, for example, in the event of any cardiovascular abnormality. Hypotension or low blood pressure may occur in a wide range of patients (for example, due to low cardiac output), especially those in intensive care or postoperative high dependency units. Measurement of cardiac output may be useful to establish a patient's initial cardiovascular state or to measure the patient's response to various therapeutic interventions. However, current methods of measuring cardiac output suffer from a variety of drawbacks. For example, arterial catheters used in measuring cardiac output are highly invasive. As another example, collection of exhaled gases may be used to measure cardiac output, but accurate collection may be difficult due to leaks around a facemask or mouthpiece.
Further, conventional non-invasive techniques suffer from drawbacks as well. For example, conventional non-invasive techniques for determining cardiac output may suffer from inaccuracy, and may often overly involve the use of guesswork or approximations in arriving at cardiac output.